Holograms have recently come into wide usage as decorative indicia due to the hologram's unique capacity to reconstruct three-dimensional images from a seemingly two-dimensional surface. Holograms are thus readily identifiable as such, even upon casual inspection, since non-holographic indicia do not create a three-dimensional virtual image. Apparatus utilized to fabricate holograms are typically complex, expensive and require sophisticated personnel to operate. Accordingly, the production of holograms is a specialty which is engaged in by companies which are suitably equipped through substantial capital expenditures.
Since holograms are difficult to make and are readily identifiable as holograms, they have become commonly employed as a means for verifying or authenticating documents, such as credit cards, driver's licenses and access cards of many types. This presumes that forgers do not generally have a means for producing holograms. Typically, the hologram is integrated with a document, e.g., an identification card, in a manner which prevents the non-destructive separation of the hologram from the document. This makes it difficult for forgers, e.g., to dissect an existing identification card and remove the hologram for the purpose of applying it to a forged identification card.
Besides holograms, producers of credit cards and the like have other techniques which may be employed to ensure the authenticity of the credit cards which are presented to stores and other vendors of goods and services. For example, credit cards now routinely include a photograph of the individual to whom the card is issued, as well as the individual's handwritten signature, both of which are incorporated into the card in a manner which prevents non-destructive isolation or removal of that element and thereby frustrates the forger. In addition to holograms and other unique identifying indicia which are incorporated into credit cards and the like, such cards frequently contain additional information, for example the identity of the issuer of the card, the account number, issue date, and various other bits of information concerning the rightful bearer and the issuer of the card.
Since credit cards and the like are intended to fit conveniently into a wallet or pocket, they are compact, thereby restricting the quantity and dimensions of the various indicia that can be displayed thereon. Thus, credit cards and other identification cards typically have discrete areas on the surface thereof which are dedicated to particular uses, for example an area which would contain the photograph of the individual bearer, an area containing written information and text, and an area for the signature of the bearer. In addition, a discrete portion of the card is normally reserved for a hologram if one is incorporated into the card.
As noted above, it is a design objective for verification apparatus and techniques for verifying cards and other documents to render the card integrated in a fashion which prevents its dissection into parts and subsequent falsified reassembly by a forger. Known techniques for accomplishing this integration function include laminating the various components into a common plastic matrix and/or adhering the respective elements to one another in a manner which prevents their non-destructive disassembly or disassociation. To this end, it is often desirable to distribute holographic indicia over a substantial portion of the surface area of a verified access card, e.g., a credit card, such that the hologram is structurally and visually associated with the entire card, rather than being relegated to a particular discrete area of the card.
In order to utilize a distributed hologram over the surface of a substrate and, at the same time, avoid obscuring other indicia which appears thereon, it has been recognized that the hologram must be at least partially transparent. Typical mass-produced holograms are phase holograms embossed in optically clear thermoplastic film as a microtexture which is then overcoated with a reflective layer. To achieve semi-transparency, strategies have been devised relating to the reflective layer. For example, U.S. Pat. No. 4,921,319 to Mallik discloses an embossed phase hologram carrying a relief pattern on one surface, which has no reflective coating thereon. Instead, an air gap is maintained between the hologram and a supporting substrate to cause incident light to be reflected from the surface relief pattern. Since the refractive index of air is significantly different from that of the plastic substrate film in which the holographic pattern is embossed, there is reflection at the interface between the relieved surface and the air gap. The Mallik '319 patent suggests that the combination of the embossed film and the air gap sets up a degree of reflection which is sufficient to read the hologram but is at least partially transparent to permit the reading of indicia that is positioned behind the hologram.
U.S. Pat. No. 5,044,707 to Mallik discloses a phase hologram employing a plurality of discontinuous aluminum dots disposed on the embossed surface as the reflective layer. Because the aluminum dots are discontinuous, a viewer may view through the hologram, i.e., between the dots, to see indicia appearing below the discontinuous holographic pattern.
U.S. Pat. No. 3,578,845 to Brooks suggests that the reflective layers of a hologram may be rendered partially transparent by varying the thickness thereof namely, from 100 angstroms to 5 microns. Such a thin reflective layer achieves the desired transparency, as well as reflectivity.
The foregoing techniques and products do, however, suffer from certain drawbacks. For instance, the air-gap phase hologram concept requires an air pocket to be maintained in the card and does not provide adequate reflectivity to efficiently reconstruct the hologram. The concept of using spaced aluminum dots to form the reflective layer is disadvantageous because the dots obscure the underlying indicia, thereby rendering the hologram less transparent. In addition, the process for producing the spacing between the dots is inconvenient and expensive. Similarly, the concept of varying the thickness of the reflective layer, while producing a semi-transparent hologram, is likewise difficult to achieve using the traditional techniques for depositing metal coatings upon an embossed polymer substrate; and, like the provision of spaced aluminum dots, it achieves increased reflectivity only at the expense of obscuring the underlying indicia.